VI A THEORY OF BIRDS’ NESTS 131 
most cases the mode of nidification (dependent on structure 
and environment) has been the cause, and not the effect, of 
the similarity or differences of the sexes as regards colour. 
When the confirmed habit of a group of birds was to build 
their nests in holes of trees like the toucans, or in holes in 
the ground like the kingfishers, the protection the female thus 
obtained, during the important and dangerous time of incuba- 
tion, placed the two sexes on an equality as regards exposure 
to attack, and allowed “sexual selection,” or any other cause, 
to act unchecked in the development of gay colours and con- 
spicuous markings in both sexes. 
‘When, on the other hand (as in the tanagers and flycatchers), 
the habit of the whole group was to build open cup-shaped 
nests in more or less exposed situations, the production of 
colour and marking in the female, by whatever cause, was 
continually checked by its rendering her too conspicuous, while 
in the male it had free play, and developed in him the most 
gorgeous hues. This, however, was not perhaps universally 
the case ; for where there was more than usual intelligence 
and capacity for change of habits, the danger the female was 
exposed to by a partial brightness of colour or marking might 
lead to the construction of a concealed or covered nest, as in 
the case of the tits and hangnests. When this occurred, a 
special protection to the female would be no longer necessary ; 
so that the acquisition of colour and the modification of the 
nest might in some cases act and react on each other and 
attain their full development together. 
Exceptional Cases confirmatory of the above Explanation 
There exist a few very curious and anomalous facts in the 
natural history of birds, which fortunately serve as crucial 
tests of the truth of this mode of explaining the inequalities 
of sexual coloration. It has been long known that in some 
species the males either assisted in, or wholly performed, the 
act of incubation. It has also been often noticed that in 
certain birds the usual sexual differences were reversed, the 
male being the more plainly coloured, the female more gay 
and often larger. I am not, however, aware that these two 
anomalies had ever been supposed to stand to each other in 
the relation of cause and effect, till I adduced them in support 
